The production of ultrapure water is essential to the fabrication of defect-free silicon chips in the microelectronics industry. Typically, producing ultrapure water involves treating water through a number of processes to remove ion contaminants. In particular, the ultrapure water (also known as deionized or high filtered water) must be virtually free of ionic contaminants, typically bringing the specific resistivity to greater than or equal to about 18.2 M.multidot.ohm.multidot.cm at 20.degree. C.
In these water purification systems, water is initially treated by a series of steps which control the pH level of the intake water, add chlorine to control bacteria growth in the water, remove particulate matter, remove added chlorine so that it does not damage delicate downstream equipment, and warm the water to about 21.degree. C. (70.degree. F.). After these initial treatment steps, the water is typically deionized in a reverse osmosis process and then degassed. The water is then further deionized by a first set of resin beds. The resin beds include beads of an ion-complexing resin which are retained in the resin beds by a screen on the exit header pipes and laterals inside the bed. The water passes through the resin beds so that it intimately contacts the resin beads to remove ion contaminants from the water. The water then passes through a plurality of 1.0 .mu.m particle pass size microfiltration modules or microfilters to remove resin particles which may have escaped the resin beds and entered the water purification system. These microfilters contain membranes of spun polypropylene or nylon which are housed in a stainless steel housing and arranged so that water enters the outer lumens of each microfilter and permeates to a common inner plenum within the housing. The water passes through the microfilters to an ultraviolet sterilization unit to control bacterial contamination and is typically stored as deionized water.
The deionized water from storage is then treated by a second set of water purification steps. These water purification steps include ultraviolet sterilization to control bacterial contamination and to convert organic materials to low molecular weight charged ions, and polishing reverse osmosis for the removal of charged ions and particulate matter. The water then passes through a final polishing system which includes another ultraviolet sterilizer and a second set of ion-complexing resin beds to remove ion contaminants from the water. Another set of microfilters is positioned downstream from the second set of resin beds to remove resin particles which may escape from the resin beds. These microfilters also have a small particle pass size (e.g. 1 .mu.m absolute and 0.1 .mu.m or 0.2 .mu.m nominal rated) and include a polyvinylidene fluoride (PVDF) lined stainless steel housing to avoid parts per trillion metals contamination. Immediately after passing through these microfilters, the water advances to a set of cross-flow ultrafilters that remove additional ion contaminants and very small particles to produce ultrapure water.
One problem that occurs in these water purification processes is that resin particles escape the ion-complexing beds and become entrained in the water flow. This "fouling" of the water occurs, to some degree, during normal system operation of the purification system. However, events such as the breakage of exit flow strainers in the resin beds can cause a sudden large release of resin particles into the flow of water. This sudden release of resin particles can blind downstream microfilters and ultrafilters and clog system apertures and instrumentation thereby reducing or stopping the flow of water in the ultrafiltration process.
As mentioned above, the conventional method of removing resin particles from the water purification system prior to ultrafiltration is to use a plurality of microfilters having a maximum particle pass size of between 0.1 and 1 .mu.m. Unfortunately, because of the small particle pass size of these microfilters, the high pressure drop through these microfilters significantly decreases the flow rate of the water through the water purification system. Therefore, ultrapure water often cannot be produced at the flow rates desired for manufacturing processes.
An additional problem associated with these microfilters is that it can be difficult to remove the resin particles trapped in the microfilters. In particular, these microfilters cannot be flushed and thus resin particles accumulate in the microfilters.
As a result, this accumulation makes it necessary to replace the microfilters in the water purification system on an annual or biannual basis. In particular, if these microfilters are not replaced, the water can become more readily contaminated and resin particles are more likely to be released into the water purification system. The replacement of microfilters causes great expense to the operation of the water purification system not only because the microfilters are expensive but because their replacement also requires that the entire purification system be shut down and opened to the atmosphere.
There is therefore a need in the art of ultrapure water systems for an apparatus and method to remove potentially damaging resin particles from the flow of water that does not cause an undesirable pressure drop, is not subject to contamination and is suitable for continuous operation and cleansing by flushing with water.